1. Field of the Invention
The invention in general relates to dental devices. More particularly, this invention relates to a device for accurate selection of standard colored dental shades to natural teeth.
2. State of the Art
The accuracy of color matching of dental prostheses (or prosthetic teeth) to natural teeth is very often considered as a crucial factor determining the quality of dental restorative work. For this reason, a dentist must make a decision as to the color to be used for dental prostheses such that the dental prostheses appear natural when implanted adjacent to natural teeth. The dentist generally refers to standard colored dental shades which he or she chooses by putting the shades against the natural teeth. The shades are typically preformed plastic or ceramic dental prostheses with certain varieties of hue, color and saturation. By holding the shade with a thin metallic holder and positioning the shade close to an adjacent referenced natural tooth the dentist decides which shade is the closest in color to the tooth. The decision is formulated in prescription to the dental lab that makes plastic, ceramic or composite dental prostheses of certain color based on standardized recipes.
The procedure of visually choosing the best matching shade often appears to be difficult. First, the color appearance depends on illumination conditions, namely light intensity and its spectral composition, or in another words, color. This problem is a common one in colorimetry that arises from the different sensitivity of the human eye (photopic and scotopic observer). In the case of natural teeth, this problem is magnified by the translucency of the teeth because illuminated light is absorbed by enamel and dentin, and then, it is scattered back. A background against which natural teeth and shades are viewed can alter the color appearance of the shades relative to the natural teeth. The color of the patient's lips and the color of gingiva contribute significantly to this background. Furthermore, the color of the natural teeth is not uniform; it typically changes in a vertical direction, from the incisal to the gingival part of the tooth and in the horizontal direction as well. The dentists try to map the color of teeth by attaching the whole dental shade to a certain part of the tooth and making a comparison. This approach is made even more difficult because the ability to choose an equivalent color is affected by the visual size of the object.
A number of devices have been proposed to assist a dentist in accurately choosing the color for dental prostheses teeth. Most of the devices aimed to replace the dentist's subjective decision by an objective measurement of color based on the illumination of teeth with polychromatic light, collecting the light that is scattered and reflected by the teeth and transforming that light into electrical signals were assumed to be adequately proportional to certain spectral components of the registered light. Determining the spectral context of light is usually performed by spectrometers that analyze continues spectra by dispersing the light with prisms or gratings or detect light at certain wavelengths by selecting it with optical filters. Measured colorimetric or spectral data from the teeth are compared with those taken from shades and stored in the memory of the device. Therefore, the closest matched shade is supposed to be chosen automatically. The photometric calorimeters that were proposed for dental application are complicated and costly to manufacture as they comprise complex integrating spheres (U.S. Pat. No. 4,881,811 to O'Brien). Simplified dental colorimeters that do not collect all scattered light, like those based on fiberoptic sensors (U.S. Pat. No. U.S. Pat. No. 4,836,674 to Lequime et al., U.S. Pat. No. 5,690,486 to Zigelbaum) or contact photoelectric calorimeters (U.S. Pat. No. 5,428,450 to Vieillefose et al) are taking light scattered by tooth differently than what appears by visual observation of natural teeth. Because of a limited aperture, the light is collected from the deeper layers of the teeth, and thus, measured spectra are affected by the thickness of the teeth and the material of the shades. Visually looking the same, teeth with differing thicknesses of enamel and different colorations of dentin will be treated by an objective photometric colorimeter as different in color. Fiberoptic proximity colorimeters are able to take signals from larger areas (U.S. Pat. Nos. 5,745,229, and 5,759,030, and 5,880,826 to Jung et al.). However, they have complex probes of multilayer fiberoptics, and, in addition, they require precise manual movement of the probe toward and away the teeth that is not convenient in the dental clinic.
An imaging device that comprises an intraoral camera, monitor, and programmable color processing means (U.S. Pat. No. 5,961,324 to Lehmann) is complicated in operation, costly, and requires extra space because of its size.
Other devices attempt to help dentist in observing the teeth and shades while giving the final decision in making the best color match to the dentist. U.S. Pat. No. 3,436,157 to Adler discloses a prismatic comparator device using a polychromatic light source which enables natural teeth to be simultaneously viewed with shades. The device, however, is bulky, comprised of four long tubes (the ends of two must be inserted in the patient's mouth to observe the patient's natural teeth). Another dental comparator device (U.S. Pat. No. 5,967,775 to Shahid) comprises a housing with a magnifying lens, a polychromatic light source and a holder of dental shades. The holder provides positioning of the shade close to the tooth, and both the tooth and shade can be viewed through the lens. This device, however, does not provide the same illumination conditions for tooth and shade as a shadow on the tooth may be created by the shade, particularly when the central part of the tooth is compared with the shade. Also, it is difficult to compare color of the tooth and shade as they typically have different sizes and shapes. In addition, the device can carry a limited number of shades because there is no space between the housing and lens, and the shades have to be manually replaced until the best color match is achieved. Furthermore, holding the housing and changing the shades requires both of the dentist's hands to be involved in handling the device. This procedure is not in convenient as the dentist needs one his hands to move the patient's lips while working with the teeth. Furthermore, in Shahid's dental comparator device, the shade prevents the lens from being constantly focused on the tooth thus requiring an exact manual focusing of the device.